Generation Mechanisms And Vertical Propagation Of Near-inertial Waves In The Western Tropical Pacific Ocean

Near-inertial waves(NIWs)contain a pronounced portion of shear energy in the internal wave field and are of great importance to deep ocean mixing.However,due to the lack of long-term and full-depth measurements,the question about how much of the near-inertial energy is actually available for the deep ocean mixing is still being debated.The accurate simulation of NIWs remains a challenge.The generation mechanisms of NIWs and the influencing factors on the vertical propagation of NIWs are largely unknown.Based on the analyses on continuous mooring data,shipboard observation,and satellite remote sensing data,we present the time-depth variations of NIWs in the western tropical Pacific Ocean,and illustrate the contribution from wind and tide to the NIWs.Here we analyzed 3-year long mooring observation to study the responses of nearinertial downward shear in the upper ocean to varying wind stress curls and sea level anomalies(SLAs).It is demonstrated that moderate(even weak)cyclone makes more contributions to enhanced shear below the pycnocline than very strong cyclone.Because very strong curl can stall the downward propagation of large shear.The large positive and negative SLAs cause the accumulation of large shear in the lower and upper parts of the pycnocline through inducing downwelling and upwelling motions,respectively.Time variation of near-inertial shear was strongly influenced by cases of large curls and interannual variation of SLA,and thus did not follow the seasonal variation of wind stress.Our analyses suggest that matched fields of wind stress curl and SLA,and well representing the ocean response to moderate cyclone are needed in simulating the role of NIWs on mixing.Based on a 3-year measurement from an observing full-depth mooring system at130°E,15°N,we find that poleward of diurnal internal tides critical latitudes,the efficiency of parametric subharmonic instability(PSI)is modified by warm core eddy(WCE)through the change of local effective inertial frequency.NIWs can be generated via PSI associated with the diurnal internal tide.The downward penetration of windgenerated NIWs can also contribute to the NIWs in the intermediate and deep layers.Warm core eddy is demonstrated to facilitate the downward penetration of NIWs.For the NIWs in the 1500 m layer,the contribution from the downward penetration of windgenerated NIWs is much larger than that from the diurnal internal tides.